The invention relates to directional boring and, in particular to a bit and method for boring through cobble formations.
Directional boring apparatus for making holes through soil are well known. The directional borer generally includes a series of drill rods joined end to end to form a drill string. The drill string is pushed or pulled though the soil by means of a powerful hydraulic device such as a hydraulic cylinder. See Malzahn, U.S. Pat. Nos. 4,945,999 and 5,070,848, and Cherrington, U.S. Pat. No. 4,697,775 (RE 33,793). The drill string may be pushed and rotated at the same time as described in Dunn, U.S. Pat. No. 4,953,633 and Deken, et al., U.S. Pat. No. 5,242,026. A spade, bit or head configured for boring is disposed at the end of the drill string and may include an ejection nozzle for water to assist in boring.
In one variation of the traditional boring system, a series of drill string rods are used in combination with a percussion tool mounted at the end of the series of rods. The rods can supply a steady pushing force to the impact and the interior of the rods can be used to supply the pneumatic borer with compressed air. See McDonald et al., U.S. Pat. No. 4,694,913. This system has, however, found limited application commercially, perhaps because the drill string tends to buckle when used for pushing if the bore hole is substantially wider than the diameter of the drill string.
Accurate directional boring necessarily requires information regarding the orientation and depth of a cutting or boring tool, which almost inevitably requires that a sensor and transmitting device (xe2x80x9csondexe2x80x9d) be attached to the cutting tool to prevent mis-boring and re-boring. One such device is described in U.S. Pat. No. 5,633,589, the disclosure of which is incorporated herein for all purposes. Baker U.S. Pat. No. 4,867,255 illustrates a steerable directional boring tool utilizing a pneumatic impactor.
At present, when underground utilities such as natural gas, potable water, or sanitary sewer pipes are placed in rock, trenches are excavated using large hard rock trenching equipment such as the Vermeer T-655, or possibly even shot using explosives. In these conditions, electric, telephone and cable TV lines are normally strung overhead along poles, mostly due to the difficulty and expense of placing them underground. Directional boring tools with rock drilling capability are described in Runquist U.S. Pat. No. 5,778,991 and in Cox European Patent Applications Nos. EP 857 852 A2 and EP 857 853 A2. Hardrock drilling normally consists of penetrating monolithic masses of solid rock such as granite in which known techniques for steering a drill bit may be employed.
However, many sites where rock strata exists include loose cobble formations. Cobble formations comprise loose rock formations including stones varying from potato size to basketball sized with voids or sand between the rocks. Drilling such formations with a directional drilling machine is problematic in that formation is not cut in the manner in which a typical rock formation is drilled. Rather, loose rocks, soil and debris must be displaced and/or compacted in order to form a bore and allow the bit and drill string to move forward though the strata in the desired direction. In such a situation, rock drilling or trenching equipment may lack the capability to displace cobble while simultaneously providing the desired steering capability. The present invention addresses this need.
The invention provides a drill bit configured for use in horizontal directional boring, and in particular horizontal drilling through cobble formation that is adapted for use with a impactor. The bit includes a spoon-shaped body and a generally cylindrical base configured for connection to the drill string. The spoon shaped body defines a conical inner surface for steering the bit in soil and an arcuate front end face that slopes rearwardly from a leading edge to a trailing edge in a circumferential direction. The arcuate front end face extends through an arc of up to 270xc2x0, and preferably from about 30xc2x0 to about 180xc2x0.
The conical face is preferably offset in that the front end face of the bit widens from the leading side surface of the bit to the trailing side surface. In this aspect, the front end face and the conical inner face may be provided with a plurality of carbide inserts to protect the bit from abrasion during the drilling operation.
In another aspect, the bit is configured with one or more fluid ports that eject drilling fluid into the bore adjacent to the bit to lubricate the bit and wash away sand and soil from between the rocks in the cobble formation. In this respect, the bit may include a longitudinal groove associated with each nozzle, the nozzle being positioned in the groove so that drilling fluid ejected from the nozzle is directed to the front end face of the bit. In one configuration, a pair of fluid ports are positioned in a pair of longitudinal grooves in the spoon-shaped body, the grooves extending from the fluid ports to the front end face to direct drilling fluid ejected through the ports into the bore adjacent to the front end face.
In one aspect the spoon-shaped body is outwardly offset in a radial direction from the base and the arcuate front end face is angled rearwardly at an angle of up to about 30xc2x0, more preferably from about 5xc2x0 to about 20xc2x0.
In another aspect, the drill bit further comprising a leading side surface and a trailing side surface. The leading side surface is narrower than the trailing side surface so that the bit acts in a wedge like manner, biting into openings between rocks in the cobble formation. In one configuration, the leading side surface is provided with a row of carbide inserts or studs to protect the surface from abrasion. In another configuration, a layer of wear resistant hard metal is applied by welding to the leading side surface and/or arcuate front end face.
These and other features and advantages are further detailed and illustrated in the following Detailed description and the Drawings.